By definition, armchairs and sofas differ from all other seating furniture, such as chairs, stools, benches and others because they are more comfortable due to big dimensions, soft cushions and ergonomic conformation aimed at allowing a comfortable and relaxing position.
The last condition is important because, as it is known, one can stay sitting in the armchair or on the sofa for long periods, even of many hours, for example, watching TV programs, reading, talking or just relaxing.
It is a well-known fact that, after a certain period of time, the human body needs to modify the assumed position, even the most comfortable, irrespective of the change of the activity performed.
Naturally, the just mentioned need becomes stronger when, though still staying sitting, one changes his activity, for example, from relaxing to reading or to watching a program or something else; as it is obvious, for each of said activities, it can be more comfortable and/or ergonomically correct to change the body position.
In order to improve the comfort, the sofas and armchairs often have supplementary cushions, either provided originally or added at a later time, which can be moved and positioned at will.
Relatively recently, the comfort has been further increased by models of armchairs and sofas available on the market, which are provided with means aimed at changing the seating arrangement, like the car seat, for example, by bigger or smaller inclination of the back and/or seat, forward translation of the seat, shift of the armrests, etc.
Some superior models are equipped also with mechanisms which, on demand, extend a footrest aimed at propping up the legs in almost horizontal position, so that the user assumes half-lying position.
However, the opportunities offered by the above mentioned devices concern ‘orthodox’ body positions, that is sufficiently straight,
whereas, when looking for a comfortable or even only temporarily satisfying position, it is non rare to assume less conventional positions, not very suitable for the normal conformation of backs, sides or armrests which, no matter how soft and pliable they are, must maintain a special profile.